There are a number of factors to consider. How much do you owe on your townhouse? Do you need the money from the sale of the townhouse as a downpayment on the condo? What would your townhouse bring in rent and would it be positive or negative cash flow situation? Do you want to be a landlord? Each of these questions and many more should be worked through before you make a decision. It might be a good idea but you have to factor in a lot of variables before you can make an informed decision.
Ditto. You should get approved first to see if you can qualify to buy without having sold you Herndon townhome. Look at the numbers, can you afford to carry both properties? Also, post your townhouse for rent on CraigsList. The posting is free, and it will be a good indicator of how much demand there is for your home should you decide to rent it. The condo fees for your purchase will likely be higher than the HOA fees for your townhouse. Keep in mind that the condo fees will be in addition to your mortgage. And find out what your townhouse is worth, it could be more or less that what you're really expecting to get.
In addition to the important information Cindy has provided, you also need to consider the current market for townhomes in Herndon. Some of the townhome communities in Herndon have a lot of short sales and foreclosures on the market and you would be competing with them. Also, have you talked with your lender regarding the condo you want to buy? Lending guidelines for condos have become much more stringent and you may find it difficult to find financing.